1. Field of the Invention
The present invention relates to a method for inhibiting the oxidation of hard metal powder in preparing sintered hard metals. More particularly, the invention relates to a method for reducing the oxidation when the hard metal powder is exposed to high temperatures in preparing the sintered hard metals using an injection molding method, by heat-treating the milled powder of the hard metal so as to remove the energy in the powder accumulated during milling.
2. Description of the Prior Art
Hard metals are meant alloys in which, for example, tungsten carbide and cobalt are mixed together. Hard metals are widely used as materials for various tools such as cutting tools, wear-resistant tools, impact-resistant tools, and so forth, since they have high hardness and high toughness. A typical fine structure of hard metal has a shape of a faceted form of tungsten carbide particulate embedded in a cobalt matrix.
A general method for preparing the sintered hard metals is as follows: Cobalt powder is initially mixed with tungsten carbide powder to give a mixture having a predetermined composition, and then the mixture is introduced into a vessel made of a hard metal or steel together with balls of a hard metal, followed by mixing and concurrently pulverizing by rotating the vessel (so called as "milling process"). For effective pulverization and mixing, acetone, alcohol or hexane is added thereto, and optionally high molecular weight additives such as paraffin are added in a small amount at the final stage of milling. A mixture (i.e., slurry) after milling is dried and granulated. The granulated hard metal powder is poured into a mold cavity having a desired shape and pressed to form a shaped compact. The shaped compact is charged into a vacuum furnace and sintered by heating [see, Hasashi Suzuki, "Hard Metal, and Sintering Hard Materials (Fundamental and Application)," Maruzen Kabushiki Kaisha (1986)].
The key to manufacturing a sintered hard metal is the milling step, which provides a sintered body having a uniformly fine structure. However, during this step, the hard metal powder, in particular, tungsten carbide powder is pulverized into smaller pieces by colliding with balls of a hard metal, and simultaneously a greater amount of energy is accumulated therein. Thus, milled hard metal powder has high reactivity with oxygen. Although serious problems do not occur when the hard metal powder thus milled is sintered by a conventional manufacturing process, such as a vacuum sintering process, oxidation of the powder cannot be avoided due to the increased activity when the powder is exposed to somewhat high temperatures in air. In order to prepare a sintered body of a hard metal using an injection molding method, the hard metal powder should be mixed with a binder of mixture of polymers by heating the mixture to a temperature higher than melting temperature of the binder; the mixture with low viscosity is injected into a mold cavity by applied pressure. The polymer binder in the molded compact should be removed, so called as "debinding", before vacuum sintering. During mixing and debinding, the milled hard metal powder having high activity will be exposed to high temperatures for long time [see, R. M. German, Powder Injection Molding, Metal Powder Industries Federation (1990)]. At that time, if there is oxygen in the atmosphere, it is necessary to add an antioxidant or to block the oxygen in order to inhibit oxidation of the hard metal powder [see, N. P. Dalskov and O. Kraemer, Injection Moulding of Hard Metal Components, Powder Metallurgy World Congress--PM '94 vol. II, p.1181 (1994); Dr. Poniatowski and G. Will, Injection Moulding of Tungsten Carbide Base Hard metals, Metal Powder Report, p. 812 (1988)].
We, the inventors have conducted extensive experimentation in order to solve the above problems. As a result, we found that when hard metal powder that has been milled in the manufacturing process is subjected to heat treatment at a temperature between about 300.degree. C. and 500.degree. C. for at least 1 hr in vacuum, its activity can be reduced due to the removal of the energy accumulated in the hard metal powder, and thus, the oxidation of the hard metal which otherwise occurs when the hard metal powder is exposed to high temperatures in preparing a hard metal through injection molding can be reduced.